TABLE 1

Fitted parameter values with single-substrate experiments


Substrate

P-gp Efflux to Apical Chamber k2a

P-gp Dissociation to Apical Membrane krb

P-gp Binding Constant KC (Cytosolic Dissociation Constant KD, Aq)c

Partition Coefficient KPCd

Steady-State Passive Permeability Coefficientse

Vmax/Km
Basolateral Transporter (Passive Transport into Cytosol)f
Apical Transporter (Passive Transport into Cytosol)f
s -1 M -1 nm/s s-1 (%)
AMP 150 2 × 106 1000 (5 μM) 200 PBA = 420 ± 50 0 (0) 0 (0)
PAB = 400 ± 50
QND 10 1 × 105 10,000 (0.1 μM) 700 PBA = 500 ± 100 0 (0) 0 (0)
PAB = 500 ± 100
LPM 1 1 × 105 20,000 (0.01 μM) 3000 PBA = 350 ± 80 100 ± 20 (40) 0 (0)
PAB = 400 ± 100
DGX 10 NDg NDg NDg PBA = 30 ± 8 30 ± 10 (70) 2 ± 1 (5)





PAB = 25 ± 5


  • AMP, amprenavir; QND, quinidine; LPM, loperamide; DGX, digoxin; ND, not determined.

  • a Estimate for the efflux rate constant k2, from P-gp into the apical chamber, given by the ratio of the fitted k2T0 for each drug and the center of the box value of T0 = 2 × 10-4 M.

  • b Estimate for the dissociation rate constant kr, from P-gp back into the inner apical monolayer.

  • c The binding constant between P-gp and the inner apical monolayer shown is the average of the best fits (> 160 for each drug concentration). Below each binding constant, we show in parentheses the appropriate dissociation constant for each drug relative to the aqueous phase, calculated as KD, Aq = 1/(KC * drug partition coefficient [phosphatidylserine (PS)/phosphatidylethanolamine (PE)/cholesterol (chol)]), the liposome mimic for the inner apical monolayer.

  • d Equilibrium drug partition coefficient to 0.1 μ m PS/PE/chol (1:1:1) liposomes. This lipid composition is a rough mimic for the inner apical monolayer, as described in Tran et al. (2005).

  • e Steady-state passive permeability coefficients measured across the confluent cell monolayer in the presence of the P-gp inhibitor GF120918. PBA and PAB are shown. The values are not always symmetric.

  • f The fits for the other transporters. Both Vmax and Km were independently fitted, but only their ratio was constant. Transport is the fraction of transport into the cytosol, B>C or A>C, due to the transporter, relative to all +GF120918 passive transport.

  • g The mass action equations only fit the product KC * KPC, which for digoxin is ∼5 × 104 M-1. This value is similar to that for amprenavir. The partition coefficient for digoxin has not been measured, so we cannot separately estimate the binding constant, KC, to P-gp or the dissociation rate constant kr = k1/KC.